Stone Density: Definition, Archimedes Method and Reference Table
Density compares the mass of a stone with that of an equal volume of water. It helps tell quartz, glass, amber or zirconia apart, and you can measure it at home with a scale and a glass of water.
Key points
| Definition | Mass density of the stone divided by that of water (no unit) |
|---|---|
| Mass density | In g/cm³, the same numerical value as the density |
| Benchmarks | Amber 1.05–1.10; quartz 2.65; diamond 3.52; corundum 4.0; gold 19.3 |
| Method | Hydrostatic weighing (Archimedes) |
| Formula | d = mass in air / (mass in air − mass in water) |
| Limit | Unreliable under 3 g, for porous stones, or for mounted jewellery |
What density measures
The mass density of a material is its mass per unit volume; for minerals it is expressed in grams per cubic centimeter (g/cm³). The density (or specific gravity) is the ratio of that mass density to that of water: it is a number without units. Since water weighs about 1 g/cm³ at room temperature, the two numbers are practically identical. Quartz with a density of 2.65 means that a piece of quartz weighs 2.65 times as much as the same volume of water.
The density of a mineral depends on two things: the weight of its atoms (lead is heavier than silicon) and their packing (how tightly they are stacked). Diamond and graphite, both made of carbon, prove it: about 3.52 for diamond, 2.1 to 2.2 for graphite.
It can be calculated from the structure
For a crystal, density can be calculated: d = (Z × M) / (N × V), where Z is the number of formula units in the unit cell, M the molar mass, N Avogadro's number (6.022 × 10²³) and V the volume of the unit cell. We carried out this calculation from published unit-cell parameters:
| Mineral | Unit cell | Calculated density | Reference table value |
|---|---|---|---|
| Halite NaCl | Cubic, a = 5.640 Å, Z = 4 | 2.16 | 2.16 |
| Quartz SiO2 | a = 4.913 Å, c = 5.405 Å, Z = 3 | 2.65 | 2.65 |
| Calcite CaCO3 | a = 4.989 Å, c = 17.062 Å, Z = 6 | 2.71 | 2.71 |
| Fluorite CaF2 | a = 5.463 Å, Z = 4 | 3.18 | 3.0 to 3.3 |
| Diamond C | a = 3.567 Å, Z = 8 | 3.52 | 3.52 |
| Corundum Al2O3 | a = 4.759 Å, c = 12.991 Å, Z = 6 | 3.99 | 3.9 to 4.1 |
| Pyrite FeS2 | a = 5.417 Å, Z = 4 | 5.01 | About 5.0 |
| Galena PbS | a = 5.936 Å, Z = 4 | 7.60 | 7.4 to 7.6 |
The values agree: density is not an arbitrary figure, it follows from the atomic architecture. The actual values of a specimen may be slightly lower (inclusions, voids) or may vary with composition.
Benchmarks: a scale of densities
| Material | Density (approx.) |
|---|---|
| Amber | 1.05 to 1.10 |
| Jet | 1.30 to 1.35 |
| Opal | 1.9 to 2.2 |
| Obsidian | 2.35 to 2.45 |
| Ordinary glass | 2.4 to 2.6 |
| Agate, chalcedony | 2.6 |
| Quartz (rock crystal, amethyst) | 2.65 |
| Pearl, coral | 2.6 to 2.8 |
| Calcite | 2.71 |
| Lead crystal | 2.9 to 3.1 |
| Jade (nephrite / jadeite) | 2.9 to 3.1 / 3.3 to 3.4 |
| Moissanite | 3.22 |
| Topaz, diamond | 3.5 |
| Garnets | 3.5 to 4.3 |
| Corundum (ruby, sapphire) | About 4.0 |
| Zircon | 4.6 to 4.7 |
| Pyrite | About 5.0 |
| Hematite | About 5.2 |
| Cubic zirconia | 5.6 to 6.0 |
| Iron-nickel meteorite | 7.3 to 7.9 |
| Native copper | About 8.9 |
| Native gold (pure) | 19.3 |
Values vary by a few percent depending on the specimen. See meteorite, hematite and amber.
Measuring density at home: hydrostatic weighing
The principle dates back to Archimedes: an object submerged in water experiences an upward force equal to the weight of the water it displaces. The apparent loss of weight therefore gives the volume. You will need a scale accurate to the centigram (0.01 g), fine thread, a glass of water, and a small support from which to hang the stone.
- Weigh the dry stone: note W air.
- Attach it to the thread and hang it in the glass of water without touching the sides or the bottom, preferably without resting it on the scale; note W water.
- Calculate: d = W air / (W air − W water).
Example. A stone weighs 12.40 g in air and 7.70 g in water. The loss is 4.70 g, so d = 12.40 / 4.70 = 2.64: this is consistent with quartz (2.65). Let us check the precision: with a 0.01 g error in the weighing in water, the result varies from 2.63 to 2.64; the uncertainty is on the order of a hundredth. By contrast, for a 1.2 g stone that weighs 0.75 g in water, an error of 0.02 g gives a result between 2.55 and 2.79: unusable.
Tips: remove the air bubbles clinging to the stone, wet it with a drop of dish soap to break the surface tension, work with water at room temperature, and repeat the measurement three times. Limits: the method is worthless for a stone mounted on jewellery, a porous stone (turquoise, hydrophane opal) that absorbs water, a hollow object, or a stone under 3 g.
The amber test in salt water
Baltic amber is very light (1.05 to 1.10). A saturated salt solution has a density of about 1.2. An amber bead therefore floats, while many glass or Bakelite imitations sink. Preparation: dissolve kitchen salt in lukewarm water, about 5 to 6 tablespoons per 250 mL, until no more salt dissolves. Place the piece in it, then rinse and dry it straight away. This test is indicative only: copal (younger resin) and some plastics also float. Avoid it for glued, porous or fragile pieces. See how to recognise real amber.
How density helps identify a stone
- Cubic zirconia versus diamond: zirconia (5.6 to 6.0) is about 1.6 times heavier than diamond (3.52) at equal volume. A zirconia the size of a one-carat diamond weighs nearly 1.6 carats.
- Rock crystal versus lead crystal: 2.65 against 2.9 to 3.1: lead crystal is noticeably heavier in the hand.
- Quartz versus glass: 2.65 against about 2.5: the difference is too small to decide; it is one clue among others.
- Lava stone (vesicular basalt): light for its size; hematite: surprisingly heavy in the hand.
A word on safety. Gemologists use very dense liquids (diiodomethane, bromoform) to classify stones. These products are toxic: never use them at home. See real or fake stone, our commitments and, for a valuable piece, certificates.
Frequently asked questions
What is the difference between density and mass density?
Mass density is expressed in g/cm³. Density is the ratio to water and has no unit. The numerical values are practically identical.
How can I measure a stone's density without laboratory equipment?
With a scale accurate to the centigram, a thread and a glass of water: d = mass in air divided by the loss of weight in water. It is reliable from about 3 g upward.
Does density alone prove that a stone is genuine?
No. It gives a clue. Several minerals have similar densities, and inclusions or voids change the value. A laboratory certificate remains the only proof.
Why does amber float in salt water?
Because its density (about 1.08) is lower than that of a saturated salt solution (about 1.2). The test is indicative: other light materials float too.
Does density change with colour?
Sometimes slightly, when the colour comes from a change in composition: iron-rich garnet is denser than magnesium-rich garnet.
Also read
The meanings mentioned belong to tradition and symbolism. They are neither promises of results nor medical advice. Back to Mineralogy for beginners.